Allogeneic hematopoietic transplantation is a curative therapy for numerous malignant and non-malignant he- matopoietic diseases that are otherwise incurable. Despite decades of intensive research, several major complications remain, including prolonged post-transplant immune deficiency and graft vs. host disease (GVHD). Gastrointestinal GVHD in particular is the predominant contributor to acute GVHD-related mortality, and dam- age to other target organs such as the thymus contributes significantly towards post-transplant immune deficiency. While much progress has been made toward understanding the immune response of the donor graft against the transplant recipient, there is little understanding of how transplant recipients respond to GVHD and its concomitant damage. Furthermore, virtually all strategies available to reduce clinical GVHD do so by limiting the donor immune system at the expense of therapeutic graft vs. leukemia/lymphoma (GVL) responses. IL-22 is a recently characterized cytokine that has been shown to protect intestinal epithelium during experimental inflammatory bowel disease. IL-22 receptor expression is restricted to non-hematopoietic cells, thus providing specificity to recipient epithelium in the transplant setting. Manipulation of this cytokine could there- fore protect epithelial tissues in transplant recipients without altering donor immunity or reducing GVL. Our preliminary data demonstrate that IL-22 is produced post-transplant by radio resistant host-derived innate lymphoid cells. These cells were eliminated during GVHD and deficiency if host-derived IL-22 led to increased GVHD morbidity, mortality, and pathology. Our data also indicate that the intestinal stem cells (ISC) necessary for nor- mal epithelial maintenance are targets of GVHD, and that IL-22 may be critical for the protection of these ISC during GVHD. Finally, our data indicate that IL-22 is critical for protecting the function of thyme epithelium post-transplant, an that IL-22 administration can eliminate thyme damage due to GVHD. We propose to test the hypothesis that IL-22 promotes survival and healing of damaged epithelium during allow- generic transplant. This project aims to: study the effects of IL-22 deficiency on GVHD and conditioning-related epithelial damage in experimental models, and study administration of IL-22 for reduction of post-transplant morbidity and mortality. Our ultimate goal is to develop a novel therapeutic strategy for prevention and treatment of GVHD. Potential therapeutic strategies will be tested in leukemia-bearing mice to ensure that GVL activity is preserved. We anticipate that these translational studies will not only lead to better understanding of immunobiology, intestinal stem cell physiology, epithelial regeneration, and GVHD pathophysiology, but will also lead to the development novel strategies to reduce epithelial damage post-transplant and improve the lives of transplant patients with both malignant and non-malignant hematopoietic disease. The specific aims are: 1: To study the effects of IL-22 deficiency on GVHD, conditioning-related damage, and post-transplant immune function. We will utilize a combination of IL-22 KO mice and IL-22 neutralizing antibody to assess the role of IL- 22 in target tissues and cells. 2: To study administration of IL-22 for reduction of post-transplant tissue damage and augmentation of post- transplant immunity. We will treat transplants recipients with systemic short-acting recombinant IL-22, induce constitutive IL-22 with an engineered expression vector, and administer IL-22 with intermediate-duration IL-22- loaded nanoparticles to test therapeutic administration strategies. The applicant, Dr. Alan Hanash, a medical oncology fellow at Memorial Sloan-Kettering Cancer Center (MSKCC) has outlined a five-year career plan that will build upon his background in immunology and clinical oncology/malignant hematology. Under the mentorship of Dr. Marcel van den Brink, a recognized leader in transplant immunology, GVHD, and immune reconstitution post-transplant, Dr. Hanash will utilize translational in vivo pre-clinical models with a combination of genetic deficiencies and cytokine administration approaches to study the role of IL-22 in reducing tissue damage and augmenting immune function after allogeneic transplant. Dr. Hanash will be mentored by an Advisory Committee of internationally recognized experts in the field. Finally, this plan is ideally carried out in the Department of Medicine and Program in Immunology at MSKCC, given its distinguished record for training physician-scientists in a rich and collaborative environment. With the support provided by the K08 award, Dr. Hanash's project will lead to the development of novel biologic insights into the relationship between lymphoid cells and stromal maintenance, as well as clinically effective strategies for promoting this maintenance during inflammatory tissue damage. In addition, the career development goal of this project is to help Dr. Hanash transition into an independent investigator with his own laboratory and R01 funding.